The present invention relates to a spark plug electrode, at whose spark exit or spark entrance area a burn-resistant noble-metal is applied.
The use of nobel-metal platinum alloy results in a substantially longer service life of he electrodes, which are exposed to very great corrosion and erosion stresses in the combustion chamber because of the conditions prevailing there. For cost reasons, no provision has been made to date to produce the electrode of a spark plug completely from noble metal. However, varied possibilities have already been found to ennoble a spark plug electrode. German Patent No. 3 727 526 describes a method of producing a spark plug for internal combustion engines, in which a noble-metal platelet on the end face of the central or ground electrode is joined by laser welding to the material of the electrode.
The use of ennobled electrodes for a spark plug is always taken into consideration when one would like to achieve very long service live and longer maintenance intervals for a spark plug. In this connection, it is known to attach these noble-metal parts to the spark plug electrodes by laser welding and resistance welding. To compensate for different expansion coefficients, it is also conventional to provide a compensating layer between the electrode and the noble-metal part, to thus produce a better and more durable bond.
In accordance with the present invention, the use of a noble-metal part, which has a smooth surface on one side and a profiled surface on the other, for welding onto the spark plug electrode has the advantage that the profiled surface, because of the smaller contact area, exhibits a high transition resistance between the noble-metal part and the electrode during the welding process. The means a correspondingly high heat energy develops in the welding zone between the noble-metal part and the spark plug electrode. In turn, the provision of a smooth surface for the noble-metal part on the side facing the welding electrode has the advantage that a small transition resistance is present here, and less heat develops at this location than at the welding position. The present construction of the welded connection largely prevents a rupture and the corrosion of the welded connection between the noble-metal part and the electrode associated with such a rupture.
Profiling in the manner that, at the beginning of the welding process, the contact areas between the noble-metal part and the central electrode exist only in the edge area of the noble-metal part offers the advantage that the heating of the noble-metal part to be welded increases starting from the edge area toward the center, and because of the star-shaped structure, the air still present between the noble-metal part and the electrode can escape via radial channels. The welding temperature has a relatively uniform characteristic over the entire cross-section, and particularly in the critical edge area, can be brought to the desired high values. In this manner, the durability of the welded connection is substantially increased, which in turn influences the service life of the spark plug in a positive manner.